Stereopsis in normal observers is most sensitive when the objects presented to the two eyes are the same size. People with different refractive errors in the two eyes (anisometropes) usually have one eye longer than the other, so the retinal images differ in size for equal-sized objects. We asked whether stereopsis is best in anisometropes when the retinal images or objects are the same. We measured stereo sensitivity for different object size ratios. Observers discriminated the orientation of a disparity-defined corrugation. Disparity noise was added to determine coherence thresholds. Threshold was best when object sizes were the same despite the differing eye lengths. Two mechanisms could account for this result. First, the retina may expand in proportion to eye length such that the number of cones sampling a given visual angle in the two eyes remains unchanged; this is the receptor hypothesis. Second, post-receptoral mechanisms may adjust for the differences in retinal-image size; this is the post-receptor hypothesis. To determine which hypothesis is a better account, we used an adaptive optics (AO) ophthalmoscope to measure linear and angular cone density in the anisometropes tested psychophysically. AO imaging was done with infrared light and dynamic wavefront correction. Images of the cone mosaic were stabilized and averaged, and individual cones identified. We could resolve cones to within ∼0.25 deg of the foveal center. Axial length, corneal curvature, and anterior chamber depth were measured using ultrasound, and those parameters were used to calculate retinal-image sizes. Angular cone density was generally higher in the longer eye. Thus, objects of the same size cover more cones in the long than the short eye, which is inconsistent with the retinal hypothesis. We conclude that anisometropes maintain fine stereopsis, despite having eyes of different lengths, via post-receptoral adaptation of the representation of the retinal images.